Types of light sources used in microscopes

There are various kinds of light source used with a microscope, each having different characteristics. Therefore, you need to select the light source according to the phenomenon you want to see. Here I explain the type and characteristics of the light source.

Tungsten lamp and halogen lamp

Tungsten lamps and halogen lamps use thermal radiation. Heat radiation is a physical phenomenon that light emits when there is heat. Since we also have a fever of 37 degrees, we are actually shining. However, since the wavelength of the light generated at 37 degrees is infrared, it can not be seen with eyes. The wavelength of the light resulting from this thermal radiation depends on the temperature. As the temperature rises, the wavelength of heat radiation’s light becomes shorter.

Using thermal radiation, we make visible light with tungsten lamp. Electric current is sent to tungsten, and the temperature raise to several thousand degrees, and thermal radiation of visible light is generated. Since the wavelength of light generated from thermal radiation is continuous, the spectrum of the tungsten lamp takes a gentle shape. Therefore, if you want to use light of a certain wavelength, you need to pass it through a filter.

The halogen lamp also has the same principle of light emission, the difference being that the gas enclosed in the lamp is halogen. Halogen has the advantage that it can reduce the burnout period of tungsten.

Mercury lamp

In the mercury lamp, light is generated by causing a discharge in a container containing mercury gas. The mechanism of light generation is as follows. First, when discharge occurs, electrons collide with mercury molecules and are excited. Ultraviolet rays are generated when returning from the excited state to the original state. The ultraviolet rays collide with the fluorescent paint around the container and emit light.

As you can see from the principle, mercury lamp is energy saving compared with tungsten lamp. Because tungsten lamp uses thermal radiation, it generates a lot of heat. On the other hand, mercury lamp excites mercury molecules directly with electrons, so energy efficiency is good.

Mercury lamp also contains light of multiple wavelengths. However, unlike a tungsten lamp, the spectrum is not gentle, and there is a particularly well-known wavelength region called a bright line. When using it with a fluorescent microscope etc., you can obtain light with high light intensity by checking the emission line position and choosing the filter to be used.

LED (Light Emitted Diode)

Semiconductors are used in LEDs. In semiconductors, we change the ease of movement of electrons by mixing impurities into silicon. Mixed with impurities that make positive charges easy to move are called P type (positive: P), and impurities that make negative charge easy to move are called N type semiconductors (negative; N for Negative).

Joining P type and N type will become a diode and current will flow in one direction. Interestingly, the positive electric charge and the negative electric charge collide with each other, and light is generated when falling into a stabilized state. This is the principle of LED.

The wavelength of the LED is determined by mixed impurities and it is characterized by being able to emit light of a specific wavelength. Also, compared with lamps, directivity is high and light tends to go straight.


Lasers are fairly special light sources compared to the above light sources and have some unique features. First, the laser goes straight. Since the angle of light is not constant in lamps and LEDs, light travels as it travels a long distance. On the other hand, in the case of a laser it is straight even if it goes a long distance. I think whether it is easy to understand when comparing the laser pointer and the flashlight. Second, the laser is coherent. Light has the nature of waves, there are light and dark with width of several hundred nm. Lasers are pretty good with lasers and can cause wave interference. With light such as lamps and LEDs, interference can not occur because the waves are not aligned. Third, the laser has the feature of a single wavelength. Only certain colors will come out. This point is similar to LED.

The laser uses the light that occurs when the state of the molecule changes. Normally, the molecule is in a stable state. When energy is given to this state, it becomes an unstable state called an excited state. Since it is unstable, it will be in a stable state after a while. At this time, light of the determined wavelength is emitted. When light is emitted at the time of emitting to excited state molecules, it amplifies and emits the emitted light.

In the laser, energy is given to the medium to create an unstable state. Then, light is generated from the medium and light is amplified in a chain. To get unidirectional light, hold the medium between the two mirrors. Then the light is reflected by the mirror and the light amplifies steadily. This is dangerous because light gathers in the medium. Therefore, let one mirror be a half mirror and release a certain amount. This light is the light of the laser.


The above is summarized as follows.

Type of light source Spectral Directional Coherence
Tungsten lamp

Halogen lamp

Gradually Low No
Mercury lamp There is an emission line Low No
LED specific wavelength High No
Laser specific wavelength Line available

Since there are many kinds of light sources, let’s choose the best one.